The invention relates to a process for continuous production of polycarbonates and to a reactor particularly suitable for this purpose.
A process for producing polycarbonate is disclosed. The process entails obtaining an oligocarbonate produced by transesterification of diaryl carbonate with dihydroxyaryl compound in the presence of catalysts and introducing the oligocarbonate in molten state into a reactor that enables continuous formation of free films at a rate higher than 10. The reactor, operating under conditions calculated to promote polycondensation, includes a horizontal cylinder equipped with a heating jacket, at least one vapor outlet, a feed nozzle an outlet nozzle, a rotatable cylindrical basket having a cylindrical perforated wall and annular discs positioned at intervals around the periphery of said basket and along the length thereof, and means for rotating said basket, the interior of said basket including no central shaft, the diameter of the cylindrical basket corresponding to about ⅔ of the diameter of the horizontal cylinder, and said discs being formed of perforated sheet metal.
Oligocarbonates are produced by transesterification of diaryl carbonates with dihydroxyaryl compounds in the presence of catalysts. Processes for the production of polycarbonates are described in the documents DE-A-1 031 512, U.S. Pat. No. 3,022,272, U.S. Pat. No. 5,340,905, U.S. Pat. No. 5,399,659, DE-A 4 312 390, U.S. Pat. No. 5,912,318, U.S. Pat. No. 5,932,683, U.S. Pat. No. 5,912,289, WO 00/26 276 and EP-A 620 240 in which the production of the intermediate stages, the oligocarbonates, is illustrated. A process for direct production of oligocarbonates is described in the German application No. 1 01 14 808.9. Further details of the melt transesterification process in general are found in the literature (see for example Hermann Schnell, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, 1964, pages 44 to 51).
Various devices for the polycondensation of oligocarbonates are described.
A helical spiral for a horizontal cylinder or a horizontal reaction pipe which spreads and conveys the melt on the reactor wall is proposed in DE-A 1 495 730 (Farbenfabriken Bayer AG). A disadvantage is that, owing to gravitation, the upper housing walls are only partially wetted and products are therefore damaged. A stable design of the spirals for large scale plant is very doubtful.
EP-A 0 711 597 (Hoechst Celanese Corporation and Hoechst AG) proposes a horizontal cylinder with a stirrer rotating therein without a centre line as a reactor device for polyester. The supporting connection elements are located on the external periphery of the stirrer. A disadvantage here is the external supporting structure, as it may lead to film bridges which may form closed chambers with the films on the stirrer blades and then impair optimal removal of the cleaved compounds. The effective treatment of higher viscosity melts is also affected as the ratio of film surface to melt volume is considerably reduced and impaired owing to the increasingly thick melt coatings.
EP-A 0 778 078 (Teijin Limited) describes a twin shaft reactor for polycondensation. Here the reaction chambers are narrow and production is difficult owing to the restricted play. Therefore, the overall dimensions are limited and are not suitable for high throughputs. Furthermore, energy is introduced via the drive and this leads to undesirable rises in temperature.
U.S. Pat. No. 5,932,683 (Asahi Kasei Kogyo Kabushiki Kaisha) describes a reactor device in which the oligocarbonate melt is distributed via a perforated disk onto a plurality of vertically attached wires and runs down them into the sump. In example 1, 50 wires 8 m in length are required for just 5 kg prepolymer per hour. It is clear that these are very complex structures which are difficult to produce on a large scale. Nitrogen is used in addition to the vacuum to aid the progress of the reaction and subsequently has to be liberated from the phenol in complex operations.
U.S. Pat. No. 5,767,224 (Bayer AG) proposes a reactor with kinematic self-cleaning for polycondensation of oligocarbonates. This reactor is described in EP A 460 466, EP A 528 210 and EP A 638 354. For high throughputs in large-scale plant this construction is very expensive and, owing to the restricted play, limited in its overall size. Furthermore, energy is introduced by the drive and this leads to undesirable rises in temperature. As inappropriate average residence times are possible, basic alkali, alkaline-earth and transition metal hydroxides, alkoxides, carbonates, acetates, boranates, phosphates and hydrides are used as catalysts in order to reduce the reaction times. However, these have an adverse effect on the quality of the polycarbonate formed.
WO-A 99/28 370 (Hitachi, Ltd.) describes a reactor or reactor device consisting of horizontal cylindrical containers which are equipped with single or twin shaft stirrers. They have good cascade characteristics and reduce the production costs compared with stirred tank cascades. A disadvantage is the external supporting structure in the single shaft stirrer, as it may lead to film bridges which may form closed chambers with the films on the stirrer blades and then impair optimal removal of the cleaved compounds. The use of twin shaft stirrers is proposed for higher viscosities. The single shaft stirrer forms excessively thick films at higher viscosities. As a result, the film surface or material exchange area is drastically reduced and the reactor does not operate effectively. In the reactor with twin shaft stirrers this deficiency is remedied by the mixing energy which is converted into kneading work. However, the drawback of a high ratio of surface area to volumetric content still exists. Another disadvantage is the energy which is introduced by the kneading work and leads to undesirable rises in temperature.
The object of all reactor devices is to provide the residence time and surface or surface renewal required for progress of the reaction. Kneading work is necessary for surface renewal, particularly at higher viscosities. The residence time required is substantially influenced by the use of catalysts and by the surface area or intensity of surface renewal over which evaporation of monohydroxyaryl compound and diaryl carbonate takes place.
In reactors with shorter residence times, an attempt is made to accelerate the reaction by using basic alkali, alkaline-earth and transition metal hydroxides, alkoxides, carbonates, acetates, boronates, phosphates and hydrides as catalysts and optionally by elevated temperatures, and to bring about sufficient surface renewal, for example by intensive kneading work. However, the use of these catalysts does not have a favourable effect on the quality of the polycarbonate formed as they remain in the product. Elevated temperatures in the presence of the above-mentioned transesterification catalysts are particularly disadvantageous to the colour of the product. The kneading work required may only be brought about by reinforced reactor constructions with high power inlet. Reactor devices of this type cannot be built in any size and this adversely affects economical throughput rates.
The object was therefore to find a reactor design which, on the one hand, permits particularly good evaporation of the monohydroxyaryl compound and optionally the diaryl carbonate by creating large melt surfaces, but, on the other hand, still allows the high throughputs desired nowadays, may be mass produced and may be operated economically.